The present invention relates to a vibration suppression structure that is used in a steering apparatus of a transport vehicle such as a motor vehicle to suppress vibration of a steering wheel having an airbag device.
During high-speed running of a vehicle or idling of the vehicle engine, vibration transmitted to the steering wheel via the steering shaft may impair driver comfort. In this regard, techniques for suppressing steering wheel vibrations have been conventionally developed and proposed. One of such techniques uses a dynamic damper that includes a weight and an elastic member, with which the weight is supported by, for example, the metal core of a steering wheel. According to the technique, when vibration the frequency of which is equal to or close to the resonance frequency of the dynamic damper is transmitted to the dynamic damper from the steering wheel, the dynamic damper resonates to absorb vibration energy of the steering wheel. This suppresses the vibration of the steering wheel.
A typical steering wheel is equipped with an incorporated airbag apparatus for protecting the driver at a collision of the vehicle. An airbag apparatus has an airbag and an inflator for supplying gas to the airbag. At a vehicle collision, the airbag is inflated with the gas supplied by the inflator. The airbag receives the body of the driver, which acts to incline forward due to the impact of the collision, thereby restricting the forward inclination to protect the driver from the impact.
Since the airbag apparatus occupies a large part of the interior space of the steering wheel, it is difficult to incorporate the above described dynamic damper in recent steering wheels. In this regard, a structure has been proposed in which an inflator is elastically supported by a support member with elastic support portions made of, for example, rubber, so that the inflator functions as a damper mass of a dynamic damper, and the elastic support portions function as springs of the dynamic damper.
If cylindrical or conical tubular members are used as the elastic support portions, the outer periphery and inner periphery both form a circle in the cross section orthogonal to its axis. The distance between the outer periphery and inner periphery, that is, the thickness of each elastic support portion is uniform in any direction (radial direction) orthogonal to the axis of the elastic support portion. Accordingly, the resonance frequency of the dynamic damper is uniform in any radial direction of the elastic support portions.
Thus, in a case in which vibration of a specific frequency in a specific direction (for example, the up-down direction) is desired to be suppressed in a steering wheel, if the direction and frequency of the vibration desired to be suppressed are one direction and one frequency, the shape (the diameter, the height and the like) of the elastic support members can be relatively easily adjusted such that the resonance frequency of the dynamic damper matches the frequency of the vibration to be suppressed.
However, when vibrations of different frequencies are desired to be suppressed in two directions orthogonal to each other (for example, the up-down direction and the left-right direction), it is difficult to adjust the elastic support members such that the dynamic damper vibrates at the frequencies desired to be suppressed both in the two directions. This is because, as described above, the outer and inner peripheries of each elastic support portion both have a circular cross section, so that the resonance frequencies in the two directions are the same.
In this regard, Japanese Laid-Open Patent Publication No. 2011-195048 discloses a structure that includes elastic support portions located between an inflator and a support member. Each elastic support portion includes ribs and a damper main body having a cylindrical or conical tubular shape. Each rib extends from a part in the circumferential direction of the outer surface of the damper main body to contact or be close to at least one of the inflator and the support member.
According to the technique disclosed in Japanese Laid-Open Patent Publication No. 2011-195048, the rib interferes with one of the inflator and the support member that contacts or is close to the rib, so that the rigidity of the elastic support portion is increased. Therefore, the rigidity of each elastic support portion in the direction in which the ribs protrudes from the outer surface of the damper main body is greater than the rigidity in other directions. The dynamic damper vibrates at a higher frequency in the direction in which the ribs protrude than in other directions. Thus, by providing ribs projecting in directions of vibrations of relatively high target frequencies, vibrations of different frequencies can be suppressed in the two orthogonal directions.
However, the ribs disclosed in Japanese Laid-Open Patent Publication No. 2011-195048 only have limited effectiveness in increasing rigidity. Therefore, the frequencies of vibrations in two orthogonal directions to be suppressed cannot be differentiated over a certain limit.